The coils or rotating electrical apparatus (e.g., a motor or generator) are insulated by lap-wrapping an insulating tape around them, placing them under a vacuum, impregnating the tape with a pressurized resin, and heat curing the resin.
A resin which is suitable for this application must, of course, be a liquid, and preferably a low viscosity liquid, so that it can easily flow around and through the insulating tape. The resin must also be solventless because any solvent which is present will be driven off during cure, leaving voids which readily break down under electrical stress. Also, the resin must be stable at room temperature for long periods of time so that a large batch of resin can be made up and stored instead of making small batches each time the resin is needed. In addition, the resin must, of course, have the mechanical and electrical properties required for coil insulation.
Until now one of the best solventless impregnating resins has been the anhydride- and phosphonium-cured epoxy resin described in U.S. Pat. No. 3,759,866. That resin has excellent electrical and mechanical properties and is easy to use in a vacuum-pressure impregnation process. However, the resin has a relatively short period of storage stability (about 90 to 100 days), and because moisture and contaminants decrease its storage stability it must be stored under anhydrous conditions.
A commercial alternative to the anhydride-cured resin is the BF.sub.3 :MEA (boron trifluoride-monoethylamine)-cured epoxy solventless resin. (See U.S. Pat. No. 2,938,880.) That resin has a much longer storage stability and is less sensitive to moisture and contaminants compared to the anhydride-cured impregnant. However, its electrical properties are not as good as desired and its tensile strength and tensile modulus at 100.degree. C are zero.